Thermal printhead

ABSTRACT

A thermal printhead according to the present invention includes an insulating head substrate (8) having an obverse surface (8a), a reverse surface (8b), a first longitudinal edge surface (8c) and a second longitudinal edge surface (8d). The surface (8a) of the head substrate (8) is formed with an array of heating dots (10a) along the first longitudinal edge surface (8c), a common electrode pattern (11) electrically connected to the array of heating dots (10a) adjacent to the first longitudinal edge surface (8c) and individual electrodes (12) extending away from the common electrode pattern (11) and being electrically connected to the respective heating dots (10a). The heating dots (10a) are selectively heated by drive elements. The common electrode pattern (11) is electrically connected to an auxiliary electrode layer (14) which covers at least the first longitudinal edge surface (8c), reverse surface (8b) and second longitudinal edge surface (8d) of the heat substrate (8).

TECHNICAL FIELD

The present invention relates to a thermal printhead, particularly to astructure provided for a head substrate of a thermal printhead.

BACKGROUND ART

Thermal printheads have been widely used for a printer of an OAapparatus such as a facsimile machine, a printer of a ticket vendingmachine and a label printer. As is commonly known, a thermal printheadselectively provides heat to a printing medium such as thermosensitivepaper or thermal-transfer ink ribbon to form needed image information.

In general, thermal printheads are divided mainly into thin film-typethermal printheads and thick film-type thermal printheads depending uponmethods of forming their heating resistors and electrode conductorlayers for example. In a thin film-type thermal printhead, a heatingresistor and an electrode conductor layer are made in the form of a thinfilm on a substrate or a glass glaze layer by sputtering for example. Onthe other hand, in a thick film-type thermal printhead, at least theheating resistor is made in the form of a thick film through such stepsas screen printing and sintering. The present invention is applicable toboth the thin film-type and thick film-type thermal printheads.

For convenience of an explanation, a structure of a typical thickfilm-type thermal printhead by the prior art is shown in FIG. 14. Thethermal printhead shown in the figure comprises an insulating headsubstrate 21 made of e.g. ceramic material. The head substrate 21 has anobverse surface formed with a glass glaze layer 22 as a heat reservoir,whereas the surface of the glaze layer 22 is formed with a linearheating resistor 23 in the form of a thick film. Further, the surface ofthe glaze layer 22 is formed with a common electrode pattern 22 havingcomb-like teeth electrically connected to the heating resistor 23 andwith a plurality of individual electrodes 25 electrically connected tothe same heating resistor 23, wherein the comb-like teeth of the commonelectrode pattern 25 divide the linear heating resistor 23 into aplurality of heating dots.

Further, the surface of the glaze layer 22 is formed with a plurality ofdrive ICs 26 to supply an electric current to the heating resistor 23,wherein each drive IC 26 is connected, via bonding wires 27, to apredetermined portion of the individual electrode 25 and to apredetermined portion of a circuit pattern (not shown) which is formedon the glaze layer 22. The drives IC 26 are enclosed together with thebonding wires 27 by a protecting resin body.

In operation, with the common electrode pattern 24 being kept at apredetermined electrical potential, the heating dots of the heatingresistor 23 are selectively actuated to generate heat by selectivelypassing a current from the drive ICs via the individual electrodes 25.As a result, predetermined images are formed on a printing medium(thermosensitive paper for example) 30 which is backed up by a platen29.

In the case of a thermal printhead having the above-described structure,a heating resistor 23 is preferably formed as close to a longitudinaledge of the head substrate 21 as possible. This is because thearrangement wherein the heating resistor 23 is formed adjacent to thelongitudinal edge of the head substrate 21 advantageously serves notonly to avoid interference of the printing medium 30 and the protectingresin body 28 with each other, but also to highten degrees ofpositioning freedom and printing quality, by holding the head substrate21 relative to the platen 29 at a certain angle.

However, if the heating resistor 23 is provided adjacent to thelongitudinal edge of the head substrate 21, spacing for formation of thecommon electrode pattern 22 is rendered correspondingly small, therebyfailing to ensure a sufficient current capacity (current passage)necessary for heat generation. As a result, the resistance of the commonelectrode pattern 24 may become disadvantageous, causing irregularitiesof generated heat between the heating dots due to a voltage drop in thelongitudinal direction of the heating resistor 23, so that printingquality will deteriorate. Particularly in the case of color printing,which has been coming into wider use recently, it is extremely importantto ensure a large current capacity, since all of the heating dots arefrequently heated simultaneously to perform so-called solid printing.

To meet such a demand, it might be conceivable to enlarge the width ofthe head substrate 21 for provision of spacing which will enableformation of a common electrode pattern having a sufficient currentcapacity between the heating resistor 23 and the longitudinal edge ofthe heat substrate 21. However, such a solution may lead to an increasein size of the head substrate 21, which is contrary to the generaldemand for a size reduction of a thermal printhead.

DISCLOSURE OF THE INVENTION

Therefore, an object of the present invention is to provide a thermalprinthead which can meet the demand for a size reduction and also canprevent quality deterioration of printed images by ensuring a sufficientcurrent capacity, even when frequently performing solid printing as inthe case of color printing for example.

To realize the object described above, according to the presentinvention, there is provided a thermal printhead comprising: aninsulating head substrate having an obverse surface, a reverse surface,a first longitudinal edge surface and a second longitudinal edgesurface; an array of heating dots formed on the obverse surface of thehead substrate along the first longitudinal edge surface; a commonelectrode pattern electrically connected to the array of heating dots onthe obverse surface of the head substrate adjacent to the firstlongitudinal edge surface; individual electrodes formed on the obversesurface of the head substrate to extend away from the common electrodepattern, the individual electrodes being electrically connected to therespective heating dots; and drive means for selectively actuating theheating dots to generate heat; wherein the common electrode pattern iselectrically connected to an auxiliary electrode layer which covers atleast the first longitudinal edge surface of the head substrate.

With the above structure, the auxiliary electrode layer electricallyconnected to the common electrode pattern serves to enlarge a currentpassage, thereby reducing the resistance to the current. Therefore, evenwhen performing solid printing where all of the heating dots aresimultaneously heated, there is hardly any occurrence of an voltage dropin the longitudinal direction of the head substrate so that the qualityof printed images will not deteriorate. Besides, since the auxiliaryelectrode layer is formed by making use of the first longitudinal edgesurface of the head substrate, there is no need to enlarge the width ofthe head substrate for formation of the auxiliary electrode layer.Therefore, the demand for a small-sized thermal printhead can besimultaneously met.

The auxiliary electrode layer may be formed to cover the reversesurface, or both the reverse surface and the second longitudinal edgesurface of the head substrate. Such an arrangement can realize anadditional enlargement of the current passage.

According to a preferred embodiment, the first edge surface of the headsubstrate comprises a step portion adjacent to the obverse surface,wherein the common electrode pattern extends onto the step portion, andthe auxiliary electrode layer also extends onto the step portion forelectrical connection to the common electrode pattern. With such anarrangement, the electrical conduction between the common electrodepattern and the auxiliary electrode layer can be further improved.

The obverse surface of the head substrate may be formed with a glazelayer having a convex portion adjacent to the first longitudinal edgesurface. In this case, if the array of heating dots is formed on theconvex portion of the glaze layer and the center line of the array ofheating dots is cause to deviate from the apex line of the convexportion toward the first longitudinal edge surface of the headsubstrate, the head substrate can advantageously contact with a platenat a large contact angle, thereby improving the printing quality. Theglaze layer may be formed to substantially entirely cover the obversesurface of the head substrate and to have a flat portion continuous withthe convex portion. Alternatively, the glaze layer may be a partialglaze layer having the convex portion alone. The head substrate and thedrive means are advantageously juxtaposed on a separate insulatingsupport board.

The heating dots may be constituted by a thin film resistor layerpatterned on the obverse surface of the head substrate. In this case,the common electrode pattern and the individual electrodes are to beformed on the resistor layer. The common electrode pattern may comprisea layer made of chromium. Further, the individual electrode ispreferably formed to comprise a first layer made of chromium and asecond layer made of a metal other than chromium, wherein the secondlayer extends toward the array of heating dots but only up to a pointshort of the extent to which the first layer extends.

On the other hand, the array of heating dots may be constituted by acontinuous thick film resistor formed in a line.

Other objects, features and advantages of the present invention willbecome apparent from the following detailed description of the preferredembodiments given with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view showing a thermal printhead according to a firstembodiment of the present invention;

FIG. 2 is a sectional view showing a head member of the same thermalprinthead;

FIG. 3 is a fragmentary plan view showing a primary part of the headmember shown in FIG. 2;

FIG. 4 is a side view schematically showing an example of usage of thethermal printhead shown in FIG. 1;

FIGS. 5a-5j show sequential steps for making the head member shown inFIGS. 2 and 3;

FIG. 6 is a fragmentary plan view showing a primary part of the headmember of a thermal printhead according to a second embodiment of thepresent invention;

FIG. 7 is a fragmentary plan view showing a primary part of the headmember of a thermal printhead according to a third embodiment of thepresent invention;

FIG. 8 is a fragmentary plan view showing a primary part of the headmember of a thermal printhead according to a fourth embodiment of thepresent invention;

FIG. 9 is a fragmentary plan view showing a primary part of the headmember of a thermal printhead according to a fifth embodiment of thepresent invention;

FIG. 10 is a fragmentary plan view of the head member shown in FIG. 9;

FIGS. 11a-11g show sequential steps for making the head member shown inFIGS. 9 and 10;

FIG. 12 is a fragmentary sectional view showing a primary part of thehead member of a thermal printhead according to a sixth embodiment ofthe present invention;

FIG. 13 is a fragmentary plan view of the head member shown in FIG. 12;and

FIG. 14 is a schematic side view showing a prior art thermal printhead.

BEET MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present invention will be described belowwith reference to the accompanying drawings. Throughout the accompanyingdrawings, same or like parts are designated by the same referencenumerals and characters.

FIGS. 1-3 show a thermal printhead according to a first embodiment ofthe present invention. The thermal printhead according to the firstembodiment, which is generally represented by reference sign A, mainlycomprises a head member 1, a support board 2 and a heat-radiating plate3.

The support board 2 made of an insulating material such as ceramic has asurface which is formed with conductor circuit patterns 4, as shown inFIG. 1. The surface of the support board 2 is also formed with aplurality of drive ICs 5 (only one of them shown) together with the headmember 1. Each of the drive ICs 5 is electrically wire-bonded partly tothe head member 1 and otherwise electrically wire-bonded to apredetermined portion of the conductor circuit pattern 4. The reversesurface of the support board 3 is fixed on the heat-radiating plate 4made of high thermal conductivity metal such as aluminum. As a result,heat which is transmitted from the head member 1 to the support board 2will be quickly dissipated into the atmosphere via the heat-radiatingplate 3.

The drive ICs 5 are enclosed by a protecting resin member 6 togetherwith the bonding wires used for electrical conduction. However, sincethe drive ICs 5 are mounted not on the head member 1, but on the supportboard 2 together with the head member 1 beside them, the upper surfaceof the head member 1 can be raised higher than the upper surface of eachdrive IC 5 (see FIG. 1). As a result, degree of projection of theprotecting resin member 6 above the upper surface of the head member 1can be rendered smaller than that of the structure of the prior artshown in FIG. 14. Correspondingly, the protecting resin member 6 maycause less interference with a printing medium 7 (such asthermosensitive paper) which is backed up by the platen (not shown)while printing.

As shown in FIG. 2, the head member 1 comprises a head substrate 8 madeof an insulating material such as ceramic, wherein the head substrate 8having a rectangular cross section comprises an obverse surface 8a, areverse surface 8b opposite to the obverse surface 8a, a firstlongitudinal edge surface 8c and a second longitudinal edge surface 8dopposite to the first longitudinal edge surface 8c. The surface 8a ofthe head substrate 8 is formed with a glass glaze layer 9 as a heatreservoir member, whereas the glaze layer 9 comprises a flat portion 9ahaving a surface generally parallel to the obverse surface 8a of thehead substrate 8, and a convex portion 9b raised above the flat portion9a.

A surface of the glaze layer 9 is formed with a resistor layer 10 in theform of a thin film. The resistor strip 10, which is divided by slits S(see FIG. 3) at a predetermined pitch, extends transversely of the headsubstrate 8 (that is, in the direction perpendicular to the longitudinaledge surfaces 8c, 8d of the head substrate 8).

As shown in FIGS. 2 and 3, the resistor layer 10 has a surface formedwith a common electrode pattern 11 adjacent to the first longitudinaledge surface 8c of the head substrate 8, and with individual electrodes12 which are spaced from the common electrode pattern 11 and extend fromthe convex portion 9b of the glaze layer 9 toward the secondlongitudinal edge surface 8d of the head substrate 8. The slits S extendto the common electrode pattern 11, separating the individual electrodes12 electrically from each other.

As described above, the individual electrodes 12 are spaced from thecommon electrode pattern 11. Therefore, the resistor layer 10 is exposedbetween the common electrode pattern 11 and the individual electrodes,wherein the exposed portions constitute heating dots (heating regions)10a, which extend in a line along the first longitudinal edge surface 8cof the head substrate 8.

In the case of the present embodiment, as shown in FIG. 3, a center lineC, which runs through the respective heating dots 10a, is caused todeviate from an apex line T of the convex portion 9b of the glaze layer9 toward the first longitudinal edge surface 8c of the head substrate 8.Therefore, as shown in FIG. 1, the head member 1 can be caused tocontact with the printing medium 7 at an inclination angle (contactangle) θ. Besides, by adjusting the deviation of of the center line Cfrom the apex line T, the contact angle θ can be made large, up to beabout 30° (or more).

The contact angle θ in question is precisely defined as the angle thehead member 1 makes with respect to the tangential line at the contactpoint of the platen (not shown). Actually, the printing medium 7 is inthe form of an arc, being backed up by the platen.

On the other hand, the contact angle θ can be made to approach zero bysetting the deviation of the center line C from the apex line T to besmall or even zero. In this case again, as already described, theprinting medium 7 does not interfere with the protecting resin body 6,since the upward projection of the protecting resin body 6 is made smallrelative to the head member 1.

It is not necessary for the convex portion 9b of the glaze layer 9 toproject upwardly beyond the flat portion 9a. The convex portion 9b canbe made in the form of an arc whose height gradually decreases from theflat portion 9a. In this case, the apex line T coincides with theboundary line between the convex portion 9b and the flat portion 9a.

In the present embodiment, as shown in FIG. 2, the heating regions(heating dots) 10a of the resistor layer 10, the common electrodepattern 11 and the individual electrodes 12 are covered with aprotecting layer 13. The protecting layer 13 serves to prevent theheating regions 10a of the resistor layer 10, the common electrodepattern 11 and the individual electrodes 12 from being oxidized byexposure to the air or from being worn away due to the contact with theprinting medium 7 (see FIG. 1).

The common electrode pattern 11 is exposed from the protecting layer 13on the side of the first longitudinal edge surface 8c of the headsubstrate 8 for electrical connection to an auxiliary electrode layer 14made of metal such as aluminum. Therefore, all portions of the commonelectrode pattern 11 are electrically connected to each other via theauxiliary electrode layer 12, thereby being kept at a same electricalpotential. In other words, the auxiliary electrode layer 12 functions asa common connecting member for all parts of the common electrode pattern11.

In the present embodiment, the auxiliary electrode layer 14 covers thewhole of the first longitudinal edge surface 8c, the reverse surface 8band the second longitudinal edge surface 8d of the head substrate 8. Onthe side of the first longitudinal edge surface 8c, the auxiliaryelectrode layer 12 extends beyond the common electrode pattern 11 toreach the protecting layer 13. Thus, the auxiliary electrode layer 12has a large area. Therefore, the current passage is enlarged, therebyserving to substantially eliminate the voltage drop across the headmember 1 in its longitudinal direction. As a result, a sufficientcurrent can be passed even when all of the heating dots 10a aresimultaneously actuated for heating (so-called solid printing), therebypreventing deterioration of the printing quality. Further, since theenlargement of the current passage is realized by forming the auxiliaryelectrode layer 12 over the first longitudinal edge surface, the reversesurface 8b and the second longitudinal edge surface 8d of the headsubstrate 8, there is no need to enlarge the width of the head substrate8, thereby enabling a compact formation of the head member 1 and thethermal printhead A as a whole.

When the head member 1 is to be mounted on the support board 2 (see FIG.1), the head member 1 can be advantageously electrically connected to apredetermined portion of the circuit patterns 4 of the support board 2by using conductive adhesive containing e.g. particulate silver, sincethe auxiliary electrode layer 14 extends over the reverse surface 8b ofthe head substrate 8. Alternatively, the head member 1 can be mounted onthe support board 2 by soldering, when the auxiliary electrode layer 14is made of e.g. aluminum (Al) and nickel (Ni)-plated.

The auxiliary electrode layer 14 may be formed to cover only the firstlongitudinal edge surface 8c of the head substrate 8. In this caseagain, when the head member 1 is to be mounted on the support board 2,the auxiliary electrode layer 14 can be advantageously electricallyconnected to a predetermined portion of the circuit patterns 4 on thesupport board 2 by soldering, since the auxiliary electrode layer 14extends toward the reverse surface 8b of the head substrate 8.

FIG. 4 shows an example of usage of the thermal printhead A having theabove structure. In this example, three thermal printheads Ay, Am, Aceach having the same structure are provided to be in facing relationwith the platen 15 to perform color printing to the printing medium 7.Of these, the thermal printhead Ay performs yellow-printing, the thermalprinthead Am performs red (magenta)-printing and the thermal printheadAc performs blue (cyanogen)-printing.

For color printing, in general, electric current to be used tends tobecome large in amount due to frequent use of solid printing. Therefore,as shown in FIG. 2, it is particularly advantageous for the head member1 of the respective thermal printheads Ay, Am, Ac to be capable ofaccommodating a large current with the use of the auxiliary electrodelayer 14. Besides, limitations to the spacing for arrangement of thethree thermal printheads Ay, Am, Ac can be reduced because of the sizereduction as already described, while allowing a large current. It isalso advantageous that the contact angle of the thermal printhead (headmember 1) relative to the platen 15 can be made large, since a largecontact angle contributes to the economy of spacing for the arrangementand also to improvement of printing quality by increasing the contactpressure against the platen 15.

Next, an example of method of making the head member 1 of the thermalprinthead A according to the above embodiment will be described withreference to FIGS. 5a-5j.

First, as shown in FIG. 5a, an alumina-ceramic master substrate 8'corresponding to a plurality of head substrates in size is prepared. Themaster substrate 8' will be divided along longitudinal division linesDL1 and transverse division lines DL2 to provide a plurality of headsubstrates.

Then, as shown in FIG. 5b, a master glaze layer 9' is formed bysintering a glass paste applied over the master substrate 8'.

Then, as shown in FIG. 5c, a groove 16 which extends into the thicknessof the master substrate 8' is formed with the use of a dicing cutter(not shown) which cuts through the master glaze layer 9' along apredetermined longitudinal division line DL1. As a result, the masterglaze layer 9' is divided into separate glaze layers 9.

Then, as shown in FIG. 5d, a flat portion 9a and a convex portion 9badjacent to the groove 16 are formed in the respective glaze layers 9 byheating the master substrate 8' at a temperature of about 850° C. forabout 20 minutes. The formation of the convex portion 9b is due to thesurface tension of the glass material which is liquidized by theheating.

Then, as shown in FIG. 5e, a tantalum nitride-based resistor layer 10 ismade in the form of a thin film having a thickness of e.g. about 0.1 μmover the glaze layers 9 by reactive sputtering. The resistor layer 10may be formed by sputtering TaSiO₂.

Then, as shown in FIG. 5f, a conductor layer 17 is formed over theresistor layer 10 by sputtering. Typically, the conductor layer 17 ismade of aluminum (Al), while it may be made of copper (Cu) or gold (Au).

Then, as shown in FIG. 5g, upon formation of slits S (see FIG. 3) byetching the resistor layer 10 and the conductor layer 17, only theconductor layer 17 is partially removed by etching for exposure ofportions of the resistor layer 10 to form heating dots 10a. As a result,the conductor layer 17 is divided into the common electrode pattern 11and the individual electrodes 12.

Then, as shown in FIG. 5h, a protecting layer 13 is formed by piling upan SiO₂ layer and a Ta₂ O₅ layer to cover the common electrode pattern11, the individual electrodes 12 and the exposed heating dots 10a of theresistor layer 10.

Then, as shown in FIG. 5i, the master substrate 8' is cut along therespective division lines BL1, BL2 by a dicing cutter (not shown) toprovide individual head substrates 8. At this time, the common electrodepattern 11 is rendered exposed on the side of the first longitudinaledge surface 8c of each head substrate 8.

Finally, as shown in FIG. 5j, to form an auxiliary electrode layer 14having a proper thickness (about 2 μm for example), conductive metal isprovided by sputtering from below to fix on the first longitudinal edgesurface 8c, reverse surface 8b and second longitudinal surface 8d of thehead substrate 8, as each head substrate 8 is being moved in thedirection indicated by an arrow X. In this case, the conductive metal istypically aluminum (Al), but copper (Cu) or gold (Au) may be usable.

In the method shown in FIGS. 5a-5j, the master substrate 8' is divided(FIG. 5i) after the protecting layer 13 is formed (FIG. 5h). However, itis also possible to form the protecting layer 13 after the mastersubstrate 8' is divided first and then the auxiliary electrode layer 14(FIG. 5j) is formed.

FIG. 6 shows a primary part of the head member of a thermal printheadaccording to a second embodiment of the present invention. The headmember of the present embodiment comprises a common electrode pattern11' having comb-like teeth, individual electrodes 12' being arranged instaggered relation with the respective comb-like teeth of the commonelectrode pattern 11' and a continuous linear thick film resistor 11a'formed to overlap on the common electrode pattern 11' and the individualelectrodes 12'. With such an arrangement, the respective heating dotsare constituted by a portion of the thick film resistor 11a' locatedbetween each pair of the adjacent comb-like teeth of the commonelectrode pattern 11'. The second embodiment is otherwise the same asthe first embodiment shown in FIGS. 1-3.

FIG. 7 shows a primary part of the head member of a thermal printheadaccording to a third embodiment of the present invention. The thirdembodiment is similar to the first embodiment shown in FIGS. 1-3 exceptonly that a common electrode pattern 11 has a continuous electrodeportion 11a formed on a glaze layer 3 (see FIG. 2) wherein thecontinuous electrode portion 11a is electrically connected to anauxiliary electrode layer 12.

FIG. 8 shows a primary part of the head member of a thermal printheadaccording to a fourth embodiment of the present invention. The fourthembodiment is similar to the second embodiment shown in FIG. 2 exceptonly that the common electrode pattern 11' made in the form of comb-liketeeth has a continuous electrode portion 11a' formed on a glaze layer 3(see FIG. 2) wherein the continuous electrode portion 11a' iselectrically connected to the auxiliary electrode layer 14.

FIGS. 9 and 10 show a primary part of the head member of a thermalprinthead according to a fifth embodiment of the present invention.

The head member 1 of the fifth embodiment comprises a head substrate 8made of an insulating material such as ceramic. The head substrate 8,which is rectangular in cross section, includes an obverse surface 8a, areverse surface 8b opposite to the obverse surface 8a, a firstlongitudinal edge surface 8c and a second longitudinal edge surface (notshown) opposite to the first longitudinal edge surface 8c. The surface8a of the head substrate 8 is formed with a strip-like partial glassglaze layer 9 as a heat reservoir only in the vicinity of the firstlongitudinal edge surface 8a. As a result, the partial glaze layer 9 asa whole is a convex. The first edge surface 8c of the head substrate 8is formed with a step portion 8e.

A resistor layer 10 is made in the form of a thin film to cover theobverse surface 8a of the head substrate 8 and the partial glaze layer9, and the resistor layer 10 further extends onto the step portion 8e ofthe first edge surface 8c of the head substrate 8. The resistor layer 10is divided into plural parts by slits S (see FIG. 10) which extendtransversely of the head substrate 8 (that is, widthwise of the headsubstrate 8).

The resistor layer 10 has a surface formed with a common electrodepattern 11 adjacent to the first longitudinal edge surface 8c of thehead substrate 8 and with individual electrodes 12 which are spaced fromthe common electrode pattern 11 and extend from the partial glaze layer9 toward the second longitudinal edge surface (not shown) of the headsubstrate 8. The common electrode pattern 11 has a continuous electrodeportion 11a which extends onto the step portion 8e of the firstlongitudinal edge surface 8c of the head substrate 8. The individualelectrodes 12 are spaced from each other by the slits S.

As described above, the individual electrodes 12 are spaced from thecommon electrode pattern 11. Therefore, the resistor layer 10 is exposedbetween the common electrode pattern 11 and the individual electrodes12, so that the exposed portions constitute heating dots (heatingregions) 10a linearly extending along the first longitudinal edgesurface 8c of the head substrate 8. Like the first embodiment shown inFIGS. 1-3, the heating dots 10a are made to slightly deviate from theapex of the partial glaze layer 9 toward the first longitudinal edgesurface 8c (step portion 8e) of the head substrate 8.

The continuous electrode portion 11a of the common electrode pattern 11extending onto the step portion 8e of the head substrate 8 iselectrically connected to the auxiliary electrode layer 14 which alsoextends onto the step portion 8e. The auxiliary electrode layer 14covers all of the first longitudinal edge surface 8c, reverse surface 8band second longitudinal edge surface (not shown) of the head substrate8. Thus, the auxiliary electrode layer 14 has a large area, therebyenlarging current passage and substantially eliminating a voltage dropin the longitudinal direction of the head member. Further, thecontinuous electrode portion 11a extending onto the step portion 8e ofthe head substrate 8 serves to enlarge a contacting area with theauxiliary electrode layer 14, thereby improving electrical connectionbetween them. Further, since the continuous electrode portion 11aextends onto the step portion 8e of the head substrate 8, the areacontacting with the auxiliary electrode layer 14 can be enlarged,thereby improving the electrical connection between them and alsoenlarging the current passage correspondingly due to the portion of thecontinuous electrode portion 11a extending onto the step portion 8e.

Although not shown, in the fifth embodiment, the heating regions(heating dots) 10a of the resistor strip 10, the common electrodepattern 11 and the individual electrodes 12 are covered with aprotecting layer 13.

Next, a preferred method of making the head member of the fifthembodiment will be described with reference to FIGS. 11a-11g.

First, as shown in FIG. 11a, an alumina-ceramic master substrate 8' isprepared which is large enough to provide a plurality of head substrateswhen the master substrate 8' is later divided along longitudinaldivision lines DL1 and transverse division lines DL2. The mastersubstrate 8' comprises a slit 18 extending along a predeterminedlongitudinal division line DL1.

Then, as shown in FIG. 11b, a groove 16 is formed in the mastersubstrate 8' along the slit 18 by a dicing cutter (not shown). Thegroove 16 will constitute the step portion 8e.

Then, as shown in FIG. 11c, partial glaze layers 9 are formed bysintering glass paste applied on the master substrate 8' adjacently tothe groove 16.

Then, as shown in FIG. 11d, resistor layers 10 are made in the form of athin film by sputtering TaSiO₂ over the partial glaze layers 9 and themaster substrate 8'. As a result, the resistor layers 10 are formed toextend over the inner walls of the groove 16 of the master substrate 8'.

Then, as shown in FIG. 11e, conductor layers 17 are formed over theresistor layers 10 by sputtering. The conductor layers 17 also extendover the inner walls of the groove 16 of the master substrate 8'.Typically, the conductor layers 17 are made of aluminum (Al), but copper(Cu) or gold (Au) may be used.

Then, as shown in FIG. 11f, upon formation of slits S (see FIG. 10) byetching the resistor layers 10 and the conductor layers 17, theconductor layers 17 are partially removed by etching to expose portionsof the resistor layers 10 to be heating dots 10a. As a result, theconductor layer is divided into a common electrode pattern 11 andindividual electrodes 12.

Then, as shown in FIG. 11g, auxiliary electrode layers 14 having aproper film thickness are formed by sputtering conductive metal (e.g.aluminum or copper) from below, as the master substrate 8' is beingmoved in the direction indicated by an arrow X. At this time, theauxiliary electrode layers 14 are formed to extend over the inner wallsof the slit 18 and groove 16, thereby being electrically connected tothe common electrode patterns 11. The film thickness of the portion ofthe auxiliary electrode layer 14 extending over the inner walls of theslit 18 and groove 16 can be controlled by the width of the slit 18.

Finally, although not shown, upon formation of protecting layers, themaster substrate 8' is cut along the respective division lines BL1, BL2to provide individual head members.

FIGS. 12 and 13 show primary parts of the head member of a thermalprinthead according to a sixth embodiment of the present invention. Thehead member of the sixth embodiment is similar to the head member of thefifth embodiment (see FIGS. 9 and 10) except for the following respects.

First, a common electrode pattern 11 is made of chromium (Cr), which hashigher thermal stability, not of aluminum or copper. The commonelectrode pattern 11 thus made of chromium is advantageous not only inthat it is easily connected to the resistor layer 10 and the auxiliaryelectrode layer 14 (made of e.g. aluminum), but also in that it ishardly deteriorated by heat.

Secondly, the individual electrode 12 is made to have a double-layerstructure which comprises a first layer 12a made of chromium and asecond layer 12b made of a different metal (e.g. aluminum or copper),wherein the second layer 12b is made to extend only up to a point shortof the extent to which the first layer 12a extends. With such anarrangement, it is advantageous that the individual electrodes 12 can beeasily attached to the resistor layer 10, and that the first layer 12ais hardly deteriorated by heat. Further, since the first layer 12a madeof chromium can be formed relatively thin and the second layer 12bextends only to a point short of the extent to which the first layer 12aextends, a printing medium backed up by the platen (not shown) can haveeasy access to the heating dots 10a, thereby improving the printingquality.

The first layer 12a of the individual electrode 12 and the commonelectrode pattern 11 are simultaneously formed by etching. The commonelectrode pattern 11 may be formed to have a double-layer structure likethe individual electrodes 12.

The present invention is described above on the basis of the preferredembodiments. However, the present invention is not limited to theseembodiments. For instance, for each method of making the head member,not only sputtering but also other methods such as CVD are applicable asa film-making method for the resistor layer, the common electrodepattern, the individual electrodes and the auxiliary electrode layer.Further, materials and configurations of the head substrate, supportboard and other composing elements are not limited to those of theembodiments. Still further, by enlarging the width of the headsubstrate, the drive ICs may be mounted on the head substrate, withoutproviding a separate support board.

We claim:
 1. A thermal printhead comprising:an insulating head substratehaving an obverse surface, a reverse surface, a first longitudinal edgesurface and a second longitudinal edge surface; an array of heating dotsformed on the obverse surface of the head substrate along the firstlongitudinal edge surface; a common electrode pattern electricallyconnected to the array of heating dots on the obverse surface of thehead substrate adjacent to the first longitudinal edge surface;individual electrodes formed on the obverse surface of the headsubstrate to extend away from the common electrode pattern, theindividual electrodes being electrically connected to the respectiveheating dots; and drive means for selectively actuating the heating dotsto generate heat; wherein the common electrode pattern is electricallyconnected to an auxiliary electrode layer which covers at least thefirst longitudinal edge surface of the head substrate; and wherein thefirst longitudinal edge surface of the head substrate has a step portionwhich is defined by a first surface extending from the obverse surfaceof the head substrate toward the reverse surface thereof and a secondsurface extending from the first surface of the step portion in parallelto the reverse surface of the substrate, the common electrode extendingonto the step portion partially in parallel to the first surface thereofand partially in parallel to the second surface thereof, the auxiliaryelectrode layer also extending on the step portion for electricalconnection to the common electrode pattern.
 2. The thermal printheadaccording to claim 1, wherein the auxiliary electrode layer also coversthe reverse surface of the head substrate.
 3. The thermal printheadaccording to claim 1, wherein the auxiliary electrode layer also coversthe reverse surface and second longitudinal edge surface of the headsubstrate.
 4. The thermal printhead according to claim 1, wherein thearray of heating dots is constituted by a resistor layer extending ontothe first and second surfaces of the step portion in direct contacttherewith.
 5. The thermal printhead according to claim 1, wherein theobverse surface of the head substrate is formed with a glaze layerhaving a convex portion adjacent to the first longitudinal edge surface,the array of heating dots being formed on the convex portion of theglaze layer, the array of heating dots having a center line whichpositionally deviates from an apex line of the convex portion toward thefirst longitudinal edge surface of the head substrate.
 6. The thermalprinthead according to claim 5, wherein the glaze layer coverssubstantially entirely the obverse surface of the head substrate and hasa flat portion continuous with the convex portion.
 7. The thermalprinthead according to claim 5, wherein the glaze layer is a partialglaze layer having only the convex portion.
 8. The thermal printheadaccording to claim 1, wherein the head substrate and the drive means arejuxtaposed on a separate insulating support board.
 9. The thermalprinthead according to claim 1, wherein the array of heating dots isconstituted by a thin film resistor layer patterned on the obversesurface of the head substrate, the common electrode pattern andindividual electrodes being formed on the resistor layer.
 10. Thethermal printhead according to claim 9, wherein the common electrodepattern comprises a layer made of chromium.
 11. The thermal printheadaccording to claim 9, wherein the individual electrodes comprises afirst layer made of chromium and a second layer made of a metal otherthan chromium.
 12. The thermal printhead according to claim 11, whereinthe second layer extends toward the array of heating dots but only up toa point short of the extent to which the first layer extends.
 13. Thethermal printhead according to claim 1, wherein the array of heatingdots is constituted by a continuous thick film resistor formed in aline.